In this study the effects of a nicotine challenge on brain activity, behavior and mood will be evaluated in two groups of healthy volunteers who have a smoking habbit: The first group will undergo a series of treatments with NicVax, aimed at producing nicotine antibodies in the body and preventing nicotine to enter the brain. The second group will receive a placebo treatment. This pre-treatment (vaccination) will take place over a 18-week period. All participants will then undergo two days of testing. On each day the subjects will perform a number of psychomotor and memory tests after a challenge with nicotine (gum) or placebo(gum). Also brain activity will be studied in an fMRI scanner. We expect to see an effect on brain activation and performance only in the placebo-vaccinated group. The Nicvax vaccinated group should show activation and performance comparable to that after challenge with placebo(gum) as in this group nicotine should not cross the blood brain barrier and enter the brain